Nano-sized molybdenum trioxide doped magnesium hydroxide and its effect on flame retardancy and smoke suppression of flexible polyvinyl chloride
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摘要: 为解决氢氧化镁(MH)阻燃效率低、抑烟效果差等问题，采用一步水热法制备出纳米氧化钼杂化氢氧化镁(MO@MH)，利用纳米氧化钼的Lewis酸催化交联作用提高MH的阻燃抑烟效率。XRD、SEM、TEM及元素分布结果表明，MoO3以小于10 nm的颗粒形式沉积在厚度10~20 nm的氢氧化镁(MH)片层中，并进一步堆积形成尺寸20~30 μm的花球状结构。阻燃性能测试结果表明，MO@MH阻燃抑烟fPVC的效率明显优于MH，MO@MH-1/fPVC复合材料的峰值热释放速率和峰值产烟率值相较于MH/fPVC复合材料分别降低了33.64%和75.16%；MO@MH/fPVC复合材料的阻燃级别达到V0级。残炭分析结果表明，MO@MH的加入使fPVC基体生成以MgO和MgMoO4为主的外层残炭，以及以石墨化碳、MgO和MgMo2O7为主的内层残炭，相较于MH/fPVC复合材料残炭的质量、致密度、完整性等方面均明显提高。+6价Mo元素通过氧化还原反应参与fPVC基体成炭过程，生成+4价Mo化合物。力学性能测试结果表明，MO@MH对fPVC基体的增韧效果优于MH，MO@MH-1/fPVC复合材料的冲击强度相较于MH/fPVC复合材料提高了32.34%，主要是因为MO@MH与fPVC基体较差的界面相容性以及复杂结构所导致的界面缺陷的存在。Abstract: In order to solve the problem of low flame retarding efficiency and poor smoke suppression effect of magnesium hydroxide (MH), nano-sized molybdenum oxide doped magnesium hydroxide (MO@MH) was prepared by one-step hydrothermal method. The flame retarding and smoke suppression efficiency of MH was improved by the Lewis acid catalytic cross-linking effect of nano-sized molybdenum oxide. XRD, SEM, TEM and element distribution results showed that MoO3 was deposited in magnesium hydroxide (MH) layers with thickness of 10~20 nm in the form of particles less than 10 nm. The MH layers were further stacked to form flower spheres with size of 20~30 μm. The results of flame-retardant tests showed that the flame retardant and smoke suppression efficiency of MO@MH fPVC was obviously better than that of MH. The peak heat release rate and peak smoke production rate of MO@MH-1/fPVC composite were 33.64% and 75.16% lower than those of MH/fPVC composite, respectively. MO@MH/fPVC composites also passed V0 rate in the vertical tests. The results of char residue analysis showed that the addition of MO@MH can produce the outer char residue mainly composed of MgO and MgMoO4, and the inner char residue mainly composed of graphitized carbon, MgO and MgMo2O7. The quality, density and integrity of the char residue of MO@MH/fPVC composites were significantly improved compared with that of MH/fPVC composite. The + 6 Mo element participated in the charring process of fPVC matrix through oxidation-reduction reactions to form +4 Mo compounds. The mechanical properties test results showed that MO@MH had better toughening effect on fPVC matrix than MH, and the impact strength of MO@MH-1/fPVC composite was increased by 32.34% compared with the MH/fPVC composite. The main reason was the poor interface compatibility between MO@MH and fPVC matrix and the existence of interface defects caused by complex structure.
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图 1 纳米氧化钼杂化氢氧化镁(MO@MH)的XRD谱图
Figure 1. XRD patterns of nano-sized molybdenum oxide doped magnesium hydroxide (MO@MH)
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图 5 MH/fPVC和MO@MH/fPVC复合材料锥形量热测试结果：(a)热释放速率；(b)总放热量；(c)产烟率；(d)总产烟量；(e) CO产率；(f) CO2产率
Figure 5. CCT results of MH/fPVC and MO@MH/fPVC composites: (a) Heat release rate; (b) Total heat release; (c) Smoke production rate; (d) Total smoke production; (e) CO production; (f) CO2 production
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图 6 MH/fPVC和MO@MH/fPVC复合材料垂直燃烧照片
Figure 6. Pictures of vertical burning tests of MH/fPVC and MO@MH/fPVC composites
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图 7 MH/fPVC和MO@MH/fPVC复合材料的燃烧残炭照片
Figure 7. Photographs of char residues of MH/fPVC and MO@MH/fPVC composites
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图 8 MH/fPVC和MO@MH/fPVC复合材料的外层残炭SEM照片及元素分布图
Figure 8. SEM images and elemental mapping of the outer layer of char residues of MH/fPVC and MO@MH/fPVC composites
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图 9 MH/fPVC和MO@MH/fPVC复合材料的内层残炭SEM照片及元素分布图
Figure 9. SEM images and elemental mapping of the inner layer of char residues of MH/fPVC and MO@MH/fPVC composites
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图 10 MH/fPVC和MO@MH/fPVC复合材料的燃烧残炭XRD谱图：(a)外层残炭；(b)内层残炭
Figure 10. XRD patterns of the char residues of MH/fPVC and MO@MH/fPVC composites: (a) Outer layer; (b) Inner layer
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图 11 MH/fPVC和MO@MH/fPVC复合材料的内层残炭Raman光谱图
Figure 11. Raman spectrum of the inner layer of the char residues of MH/fPVC and MO@MH/fPVC composites
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图 12 MH/fPVC和MO@MH/fPVC复合材料的内层残炭XPS谱图
Figure 12. XPS spectrum of the inner layer of the char residues of MH/fPVC and MO@MH/fPVC composites
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图 13 MH/fPVC和MO@MH/fPVC复合材料的力学性能：(a)拉伸应力-应变曲线；(b)冲击强度
Figure 13. Mechanical properties of MH/fPVC and MO@MH/fPVC composites: (a) Tensile stress-strain curves; (b) Impact strength
表 1 MO@MH材料的元素组成
Table 1 Elemental composition of MO@MH
Element MO@MH-5 MO@MH-2 MO@MH-1 Wt.% At.% Wt.% At.% Wt.% At.% O 46.49 67.82 41.39 68.99 39.91 70.31 Mg 26.28 25.56 17.68 19.64 13.69 16.07 Mo 27.23 6.62 40.93 11.37 46.40 13.62 Notes: Wt.% is the weight percent; At.% is the atomic percent. 
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表 2 MH/fPVC和MO@MH/fPVC复合材料LOI及锥形量热相关参数
Table 2 Parameters of the LOI and CCT result of the MH/fPVC and MO@MH/fPVC composites
Sample LOI/% pHRR/kW·m−2 THR/MJ·m−2 pSPR/m2·s−1 TSP/m2 pCO/% pCO2/% Residue/wt.% MH/fPVC 29.7 210.83 48.42 0.0942 11.07 0.048 0.414 35.34 MO@MH-5/fPVC 34.0 167.23 42.80 0.0408 8.60 0.026 0.343 45.72 MO@MH-2/fPVC 34.5 186.96 42.67 0.0474 7.62 0.034 0.394 43.32 MO@MH-1/fPVC 31.0 139.91 39.31 0.0234 4.93 0.025 0.297 46.21 Notes: pHRR is the peak of the heat release rate; THR is the total heat release; pSPR is the peak of the smoke production rate; TSP is the total smoke production; pCO and pCO2 are the peak of the CO and CO2 production.
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表 3 MH/fPVC和MO@MH/fPVC复合材料力学性能
Table 3 Mechanical properties of the MH/fPVC and MO@MH/fPVC composites
Sample Tensile strength/MPa Tensile modulus of elasticity/MPa Nominal strains at break/% Impact strength/(kJ·m−2) MH/fPVC 16.32±0.32 7.82±0.12 193.38±10.28 53.99±2.43 MO@MH-5/fPVC 14.62±0.84 9.65±0.64 164.97±13.01 59.14±5.25 MO@MH-2/fPVC 13.97±0.58 8.97±0.46 129.08±10.61 64.48±6.45 MO@MH-1/fPVC 12.77±1.42 9.62±0.38 119.58±16.31 71.45±1.43
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目的
氢氧化镁(MH)是一种重要的环保型无卤阻燃剂，在聚合物复合材料领域有良好的应用前景。然而，受限于物理作用(分解吸热、水蒸汽稀释、氧化镁阻隔效应等)的阻燃机制，MH的阻燃效率较低、所需添加量大，对复合材料的力学性能及加工流动性能影响较大。为了提高MH阻燃软质PVC(fPVC)的效率、减少其添加量，本文利用纳米氧化钼的Lewis酸催化交联成炭作用与MH形成协同效果，制备出高效阻燃抑烟的纳米氧化钼杂化MH(MO@MH)材料，探究其阻燃机制。
方法采用一步水热法制备出三种不同配比的MO@MH，将其用作fPVC的阻燃抑烟剂。通过LOI、锥形量热和水平垂直燃烧评价了三种MO@MH对fPVC阻燃抑烟效果的影响；采用SEM、XRD、XPS、Raman等详细分析了复合材料的燃烧残炭，推断MO@MH阻燃fPVC的机制；通过拉伸和冲击性能测试评价MO@MH对fPVC力学性能的影响。
结果XRD、SEM、TEM及元素分布结果表明，氧化钼以小于10 nm的颗粒形式沉积在厚度10~20 nm的氢氧化镁(MH)片层中，并进一步堆积形成尺寸20~30 μm的花球状结构。阻燃性能测试结果表明，MO@MH对fPVC的阻燃、抑烟效果优于MH，且随着氧化钼含量的增加，阻燃抑烟效果越好。MO@MH-5/fPVC、MO@MH-2/fPVC和MO@MH-1/fPVC复合材料的LOI值分别达到了34.0%、34.5%和31.0%，垂直燃烧级别均达到UL 94的V0级。其中，MO@MH-1/fPVC复合材料的峰值热释放速率、总放热量、峰值产烟率、总产烟量、CO和CO峰值产率相较于MH/fPVC复合材料分别降低了33.64%、18.81%、75.16%、55.47%、47.92%和28.26%。MO@MH-5/fPVC、MO@MH-2/fPVC和MO@MH-1/fPVC复合材料的在燃烧过程中均表现出明显的膨胀效应，残炭量分别为45.72、43.32和46.21 wt%，相较于MH/fPVC复合材料的35.34 wt%有着明显增多，且强度显著增强。SEM结果表明，MO@MH的加入能够有效改善内层残炭的致密性与完整性。EDS和XRD结果表明，MO@MH的加入使fPVC基体生成以MgO和MgMoO为主的外层残炭，以及以石墨化碳、MgO和MgMoO为主的内层残炭。拉曼光谱数据表明，MH/fPVC复合材料内层残炭的/值为2.67，而MO@MH-5/fPVC、MO@MH-2/fPVC和MO@MH-1/fPVC复合材料内层残炭的/值分别降低至2.17、1.87和1.65。XPS结果表明，+6价Mo元素通过氧化还原反应参与fPVC基体成炭过程，生成+4价Mo化合物。最后，力学性能测试结果表明，MO@MH对fPVC基体的拉伸弹性模量和冲击强度的提升效果优于MH，MO@MH-1/fPVC复合材料的拉伸弹性模量和冲击强度相较于MH/fPVC复合材料分别提高了23.40%和32.34%。
结论纳米氧化钼杂化MH后，随着氧化钼含量的增加，MO@MH/fPVC复合材料的阻燃抑烟性能逐渐提高。阻燃抑烟性能的提高主要源于纳米氧化钼的Lewis酸催化交联作用，促使fPVC基体生成了结构更加致密、数量更多、石墨化程度更高的炭层，从而更加有效地阻隔了热量、氧气以及可燃性挥发物的传递。MO@MH/fPVC复合材料的拉伸弹性模量的提高则归因于MO@MH更加复杂的三维结构以及更强的刚性。
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